A customized computer program (MRIView) is described for visualizing and quantifying complex blood flow patterns in major vessels, using nongated and cardiac-gated three-dimensional (3D) velocity data obtained with MR velocity-encoded phase pulse sequences. Streamlines, arrows, and particle paths (collectively referred to as "paths") can be computed interactively, using both forward and backward time integration of the velocity field. The program provides interactive cross-sectional and 3D perspective visualization of the paths, with quantification and statistical analysis of average speed, through-plane velocity, cross-sectional area, and flow. Normal flow patterns in the carotid artery, basilar artery tip, ascending aorta, coronary arteries, descending aorta, and renal arteries, as well as abnormal flow patterns in basilar tip aneurysms, have been investigated. The program revealed flow patterns in these regions with features that are well known from Doppler ultrasound and other features that have not been reported previously. The association between specific abnormal flow patterns and development of atherosclerosis suggests that particle paths can be used to assess risk of plaque formation and progression, as well as to evaluate flow dynamics and vascular patency before and after vascular interventions.